The purpose of this project is to establish the system of assay of in vitro malignant transformation of human cells by chemical carcinogens. The normal diploid human fibroblasts were transformed by chemical carcinogens. Transformed cells were characterized by morphological changes in culture, growth ability in soft agar, and tumor formation in nude mice. Some conditions required for the development of the transformation were defined. BIBLIOGRAPHIC REFERENCES: Kakunaga, T.: The transformation of human diploid cells by chemical carcinogens. In Watson, J.D. (Ed.): The Origin of Human Tumor. New York Cold Spring Harbor Laboratory, 1977 (in press).